An atomic force microscope (AFM) is a deflection detection optical device in which, in a common implementation, forces are measured by means of a cantilever that deflects when forces act on it. The deflection of the cantilever is sensed by a detection system, commonly by focusing an incident beam as a spot onto a cantilever and directing the reflected beam onto a segmented detector. Conventionally, a two-segment detector, such as a split photodiode, is used. Whereas, only two segments are usually used to sense either vertical or horizontal cantilever deflections, the photo diode can have four segments for measuring cantilever deflections in both directions. Initially the beam spot must be positioned so as to be approximately equally incident on each of the segments. The deflection of the cantilever is detected as a difference between the incident powers on each segment.
In the approximately twelve years since its invention, the AFM has become more and more advanced, measuring smaller and smaller forces and utilizing smaller and smaller cantilevers. This has introduced problems relating to forming appropriate incident light beam spots on such very small cantilevers and in detecting cantilever deflection during scanning as well as when different cantilevers are brought into position for different uses.
In addition, a fundamental limit is the source of noise in the AFM, generally resulting from thermal noise of the cantilever. With the use of smaller cantilevers, this noise source can be reduced such that very small forces can be measured in principle. However, with smaller forces, the deflections of the cantilever become smaller and the detection noise becomes more and more significant. Therefore, it is important to have a reliable, low-noise detection system.
The present invention provides a high sensitivity deflection detection device that uses a light spot, with improvements to reduce detection noise and thermal drift and allow for higher signal-to-noise ratios of measurements of the beam deflection. Such a device is illustrated by an atomic force microscope. The invention provides improvements involving the incident beam as well as a detection system. In particular, a high sensitivity atomic force microscope is provided including one or more of the following: means in the path of the incident beam for adjusting the size, shape and/or power of the incident beam spot, means for moving the incident beam spot with movement of the cantilever whereby to maintain the position of the spot on the cantilever, means for reducing the signal to noise ratio of the optical detector in which different gains are applied to different segments of the optical detector, and means for regulating the temperature of the environment of the atomic force microscope to limit thermal drift. The invention incorporates a variety of strategies that individually, and in particular in combination, provide higher sensitivity to the atomic force microscope.
To adjust the size of the incident beam spot, one can place zoom optics in the path of the incident beam having a plurality of lenses chosen and arranged to provide an adjustable focal length. This can be combined with a viewing system that is confocal with the beam spot on the cantilever. One can selectively place one of a plurality of cantilevers in the path of the incident beam and, by placing a selected cylindrical lens in the light path, one can fit the spot to the selected cantilever.
With collimated light, one can place an adjustable beam expander in the path of the incident beam, the expander having at least one lens that diverges or converges the collimated light, and at least one lens that re-collimates the diverged or conversed light beam to a different size beam. Alternatively, when using a plurality of cantilevers, a removable and interchangeable lens can be placed at a distance from each particular cantilever to produce different size focus spots for respective selected cantilevers.
To adjust the power of the incident beam spot, a mask of variable optical transmittance can be inserted in the path of the incident beam, preferably in an image plane with respect to the plane of the cantilever. The mask may be patterned to produce a desired irradiance distribution of the incident beam spot on the cantilever.
Other aspects of the invention provide control means for moving the incident beam spot with movement of the cantilever to maintain the position of the spot on the cantilever. In particular, a change in position of the cantilever is determined which is compared to the position of the spot and the position of the spot is then adjusted until it corresponds to the position of the cantilever.
Detection noise is reduced by providing the segments of the optical detector with different gains optimized to maximize the signal to noise ratio of the particular measurement. The segments are arranged in an array, in a particular embodiment as the component of a CCD chip. A plurality of pre-amplifiers are associated with respective segments, and summing means are provided to receive the pre-amplified outputs of the segments. In one embodiment, the different gains are applied to the respective pre-amplifiers. In another embodiment, the different gains are applied to the summing means.
Thermal drift of the instrument is limited by regulating the temperature of the environment, specifically by closing the atomic force microscope in a housing.